Zero turning radius (“ZTR”) mowers have at least one independently powered drive wheel on each side of a frame. Hydrostatic transmissions transmit power to each of the left and right drive wheels, either in forward or reverse. The pair of hydrostatic transmissions, or dual hydrostatic transmission, may be driven by an internal combustion engine. The independent rear drive wheels allow the ZTR mower to turn on a vertical turning axis. The vertical turning axis may be centrally located between the pair of hydrostatic transmissions.
ZTR mowers have frames with left and right longitudinal frame members supported on a forward end by front wheels and extending rearwardly to support an internal combustion engine. A mower deck may be suspended between the front and rear wheels. A seated operator may use left and right control levers or other steering controls to control the pair of hydrostatic transmissions driving the left and right rear drive wheels.
Internal combustion engines on ZTR mowers are mounted on the mower frame or platform behind the operator seat and rear wheels. Many ZTR mowers have internal combustion engines with vertical drive shafts. Pulleys on the vertical drive shafts drive belts that transmit power from the engine to the pair of hydrostatic transmissions with vertical shafts, and transmit power from the engine to pulleys on vertical shafts of a mower deck. Examples of ZTR mowers with vertical shaft engines include U.S. Pat. No. 5,816,034 for “Belt Design for Mower,” U.S. Pat. No. 6,952,913 for “Adjustable Belt Pulley System,” U.S. Pat. No. 7,596,934 for “Lawn Mower with Belt Drive System,” U.S. Pat. No. 7,717,219 for “Unitary Rear Frame for Mounting Engine, Hydrostatic Transmission, and Other Components to Mower,” and U.S. Pat. No. 7,913,479 for “Two-Pulley Belt Tensioning Mechanism.”
Alternatively, some ZTR mowers have internal combustion engines, typically with higher horsepower, having horizontal drive shafts oriented along the fore-aft axis of the mower. To transmit power from a horizontal shaft engine to vertical shaft pulleys of the hydrostatic transmissions and mower deck, mule drives are typically used. Mule drives include one or more belts that are driven by a pulley on the engine's horizontal drive shaft, twisting the belt to engage pulleys on vertical shafts of the hydrostatic transmissions and mower deck. Examples of ZTR mowers with horizontal drive shafts and mule drives include U.S. Pat. No. 5,865,020 for “Lawn Mower Having a Lower Center of Gravity,” U.S. Pat. No. 6,651,413 for “Multiple Belt Mule Drive Apparatus and Systems/Vehicles Using Same,” U.S. Pat. No. 7,427,247 for “Mower Incorporating a Mule Drive,” and U.S. Pat. No. 7,665,284 and U.S. Pat. No. 7,856,799 for “Belt Drive for Lawn Mowers.”
ZTR mowers with horizontal shaft engine are typically longer than ZTR mowers with vertical shaft engines, because the engines and mule drives extend farther behind the operator seat. Horizontal drive shaft engines and mule drives may extend rearwardly outside the ZTR mower's turning radius or turning circle, reducing the maneuverability of the ZTR mower, especially when mowing close to obstacles. A ZTR mower is needed with a horizontal drive shaft engine, but without the engine or mule drive extending rearwardly outside the ZTR mower's turning radius or turning circle. Additionally, mule drives require substantial distance and space between the horizontal and vertical shafts in order to twist the belts. A ZTR mower is needed with a horizontal engine that does not require a mule drive.
Gearboxes may be used to convert a horizontal shaft drive to a vertical shaft drive. However, conventional gearboxes have gaseous pressure that builds up inside the gearbox as temperatures increase. In the past, spring-loaded vents have been used to vent gearboxes to atmosphere, with venting parts extending out through the gearbox wall. These vents present packaging issues, and may require cutting or modifying the mower frame next to the gearbox wall where the vent is located. A ZTR mower is needed with a gearbox vent having fewer parts and lower cost than conventional spring-loaded gearbox vents, lower risk of damage during assembly or operation, and reduced or eliminated packaging issues.